1. Field of the Invention
The present invention relates in general to nanoparticles exhibiting luminescence such as photostimulated luminescence or photoluminescence and optical switching processes based upon such properties, in more particular, the use of such photostimulated luminescence exhibiting nanoparticles and switching nanoparticle for optical storage apparatuses and sensors as well as methods of making and using same.
2. Background of Art
The use of nanoparticles or quantum dots exhibiting photostimulated luminescence (“PSL”) for the storage of digital information offers a tremendous advance in the digital storage art. Traditionally, cost-effective storage of information required ultrahigh packing densities as well as inexpensive self-assembling techniques and fast methods for writing and retrieving information. Semiconductor quantum dots (QDs), which involve a few thousand atoms, offer an attractive path toward achieving these goals. Single-electron storage has been suggested in the art as a possibility with quantum dots. Charge storage devices based on the resistivity change of a two-dimensional electron gas located near a layer of self-assembled QDs have been demonstrated by those skilled in the art at low temperature. A memory cell based on field-effect tunable lateral potential modulation in the plane of a semiconductor quantum well and acoustically driven storage in quantum wells have also been proposed and demonstrated by those skilled in the art. The storage characteristics of these approaches are limited to short duration (several hundred microseconds) at very low temperatures.
Optical storage and retrieval of excitons in semiconductor self-assembled quantum dots have been demonstrated but the storage times were only on the order of several seconds at very low temperatures. Such a short storage time is not long enough for practical application. Herein is claimed and disclosed an optical storage apparatus and methods of use utilizing nanoparticles exhibiting photostimulated luminescence and/or optical switch process at room temperature with a storage longevity on the order of multiple hours.
Photostimulated luminescence is a process in which trapped charges are released by photons to produce luminescence through recombination and has been previously studied extensively in the area of X-ray radiology. The use of photostimulable BaFBr:Eu2+ phosphors for X-ray storage and imaging has proven to be one of the most successful detectors in digital radiography. The photostimulated luminescence mechanism of BaFBr:Eu2+ involving X-ray irradiation proposed by Takahashi et al. assumed that, during X-ray irradiation, the Eu2+ ions are partly ionized into their trivalent charge state (Eu3+) and the liberated electrons drift via the conduction band to form F centers. Upon subsequent photostimulation, the electrons are released from the F centers into the conduction band and thereafter recombine with Eu3+ ions to produce the photostimulated luminescence of Eu2+ at 390 nm.
A general erasable optical storage apparatus using photostimulated luminescence is shown displayed in FIG. 1. As shown in FIG. 1, the writing light can be either ultra-violet (UV) or blue or any other light having energy higher than the energy gap of the host materials (i.e. the writing light is variable and will depend on the energy gap of the host material). The reading light can be visible or infrared (IR) light, the choice of reading light is also variable and depends on the trap depth of the host material. Semiconductors such as MgS, CaS, SrS, and SrSe doped with rare earth elements such as Ce, Sm, and Eu have been previously considered for optical storage and dosimetric applications. These materials possess a high sensitivity for radiation energy storage and a high PSL efficiency under IR stimulation. However, they suffer from significant fading at room temperature due to their narrow band gaps and the fact that their shallow trap depths do not efficiently store energies. The PSL nanoparticles of the presently claimed and disclosed invention overcome such limitations in the art.
An application of PSL phosphors is medical imaging storage. Such PSL phosphors must posses the properties of high density, high brightness, short decay lifetimes, suitable emission energy and stimulation energy and low light scattering. The energy structure in the phosphors is critical to the effective operation of the detector. The trap depth to the conduction band must be small enough so that stimulation with laser light is possible, yet sufficiently large to prevent random thermal release of the electron from the trap. Generally, the trap depth should be larger than 0.5 eV to prevent thermal release or fading at room temperature. In addition, the wavelength separation between the stimulation light for reading (corresponding to the color center absorption band) and the monitored emitting light of the recombination center should be sufficiently large so that noise signals due to reading light reflection are avoided. The phosphors may have only one type of trap to thereby reduce signal loss due to electron migration among different traps. Currently in the art, there is no PSL phosphor that meets all of these stated requirements. The photostimulable phosphor used in commercial X-ray imaging system is BaFBr:Eu2+. Yet, the BaFBr:Eu2+ system suffers from poor resolution due to the long decay lifetime of Eu2+ emission (0.8 μs) as well as scattering of the stimulating laser light from the plate-like polycrystals. In addition, the hygroscopic nature of this phosphor limits the stability of the system.
Recently, it was disclosed that CdSe nanoparticles can transfer in a metastable ‘dark’ state by heating, in which the luminescence is quenched. Further, the luminescence returns when the material is exposed to light. Such a nanoparticle capable of reversible change is also useful for optical storage and sensor applications. Contrary to this disclosure, another group observed that it is possible to turn on fluorescence in thin layers of silver oxide, which are initially non-fluorescent, by exposing them to a light source having wavelengths less than 520 nm. Once photoactivated, the silver oxide layers exhibit multicolored fluorescence under excitation by both blue (450 to 480 nm) and green (510 to 550 nm) light. Blue excitation causes the layer to fluoresce in multicolored hues, but only intermittently. Green excitation, however, results in brighter and steadier red fluorescence. Such “caged” fluorescent nanoparticles of silver may be rapidly switched on and used, for example, as nanoscopic optical storage elements or as probes in living systems. Similarly, an additional group showed photosensitivity in Ag+ doped phosphate. Presently, claimed and disclosed is an optical storage apparatus(es), sensors, and methods of making and using same based on the reversible optical processes of rare-earth ion doped nanoparticles, such as Y2O3:Eu3+ nanoparticles, at room temperature.